Methods of accelerating a body typically involve contact between the body to be accelerated and an external body, or the discharge of reaction mass from the body to be accelerated. For example, an automobile is accelerated by applying motive power from the engine to drive wheels in contact with a paved or unpaved surface. Friction between the wheels and the surface allow the rotation of the wheels to produce substantially linear acceleration of the vehicle. Thus, the vehicle is put in motion by a force applied against an external mass. As another example, a rocket engine, ion engine, or other engine typically used to propel a satellite or other spacecraft discharges mass (reaction mass) during operation. Such mass may be fuel, combustion products, electrons, ions, or any other type of mass. A portion of the accelerating mass is itself accelerated in a direction opposite the desired direction of motion and discarded, resulting in a reaction force acting on the satellite or other spacecraft.
Because propulsion devices consume and/or discharge mass in order to produce acceleration, the ability of a propulsion device to accelerate a body at a particular rate is constrained by the amount of mass available for consumption or discharge. For example, the station-keeping function of a satellite is often performed with small rocket motors utilizing a fuel such as hydrazine. When the fuel for those station-keeping motors is exhausted, the satellite may drift out of control, ending its useful life even though the components of the satellite, such as the electrical power source, may have significant useful life remaining. Thus, the amount of fuel available for use by a propulsion device may have a direct impact on the useful life of a satellite, vehicle, or other body utilizing that propulsive device.
Further, the amount of fuel or reaction mass available for use by a propulsive device can limit the maximum velocity that can be attained by a body. For example, a spacecraft utilizing an ion propulsion system can accelerate substantially continuously to a final velocity that is attained when the supply of ions utilized for thrust is exhausted. At that point, the spacecraft continues to travel at the final velocity. Thus, the maximum velocity of a spacecraft is limited by the amount of fuel or reaction mass that the spacecraft can carry.